Phosphogypsum (PG) is a waste product of the phosphate fertilizer industry from produced by the reaction of phosphate with sulfuric acid. Approximately 3 Mt are produced annually in Israel alone and up to 280 Mt globally. PG contains approximately 90% calcium sulfate dihydrate (gypsum). However, PG is contaminated with small amounts (<1%) of phosphoric acid left over from industrial processing, other chemical contaminants (most notably fluorides and regulated metals), rare earth elements, and radionuclides. Radium (Ra) is present at 600-1000 Bq/kg, which exceeds the legal limit allowed in construction materials in most developed nations. The use of PG as a construction material is currently impractical because of these contaminants. Indeed, approximately 82-85% of PG produced worldwide is dumped in stacks (i.e. landfill piles). As such, stacks pose an environmental and human health hazard primarily due to the contaminants, particularly Ra, and take up an increasing amount of potentially usable land.
Several solutions for the purification of PG have been tested successfully on a laboratory scale. One approach is to create phase changes in the calcium sulfate between the hemihydrate (HH) and dihydrate (DH) states. This allows purification of the PG from phosphates, but not from radionuclides, including Ra. Another approach is separation of the PG grains by size, as smaller grains generally contain a higher percentage of contaminants. Indeed, Ra in the fine (particle size<30 microns) PG fraction contains about 40% of the total Ra.
Radionuclides can also be removed by sulfuric acid extraction. However, due to their cost and inefficiencies, these and other approaches have not been successfully extended beyond the laboratory. As such, these approaches have failed to produce a breakthrough in the industry.
All known approaches of removing impurities, such as heavy metals and radionuclides, from PG are irreconcilable with the economy and the environment. In particular, the chemicals and the products of their neutralization, employed and generated by known approaches continuously pollute the environment. Moreover, no single approach can achieve chemical and radioactive purification simultaneously. Importantly, all current approaches yield, inter alia, a brine of contaminants, which cannot be utilized at present. In almost all situations these waste streams are discharged into the environment. Suffice it to say, despite significant efforts to develop a process that allows utilization of PG, there still exists a need for a method to remove impurities from PG and allow for its utilization that is practical and environmentally friendly on an industrial scale.